Ok, here's breakdown how OZ launches its X11 system:
1. /etc/init.d/gpe-dm launches /etx/X11/Xserver
Xserver is a shellscript which sets mutliple paramters for the real X server, like rotation, DPI and input device and the selects the correct xserver binary as well. For OZ that would be kdrive-fbdev.
2. After the xserver is running, all scripts in /etc/X11/Xinit.d are sourced and executed.
Various things are configured at this stage: correct xmodmap (x11 keymapping), TS calibration if required and the mouse pointer is made invisible.
The last script launches gpe-login, the GPE login manager
3. After gpe-login is used to login as a user, a new xserver running with the permissions of $USER is launched and all scripts in /etc/init.d/Xsession.d are executed
At this stage all user-space stuff required for GPE operation is launched under the UID of $USER
(ipaq-sleep, gconfd, keylaunch etc)
The very last script in Xsession.d launches the GUI:
# cat 99xWindowManager
#!/bin/sh
exec x-window-manager
#
x-window-manager is a symlink handled by update-alternatives and points to the session file used to bring up the GUI.
Reroute that script to something else to launch another WM.
The typical GPE session consists of:
matchbox-panel
matchbox-desktop
matchbox-wm
You may replace the components with anything you want but keep in mind that a working X environment consists of many many small things that work together.